1. Field of the Invention
The present invention relates to a magnetic element such as a transformer used as a band pass filter for example.
2. Description of the Related Art
Among transformers which are kinds of magnetic elements, there is a type that functions as a so-called band pass filter (band filter), which only passes signals in a desired frequency band (for example, in the vicinity of 13 kHz). In this type of transformer, it is required to set an impedance to a desired value and have a characteristic such that among signals inputted to a primary side, a high frequency signal equal to or higher than a predetermined frequency and a low frequency signal equal to or lower than a predetermined frequency are attenuated on a secondary side.
Incidentally, the impedance is in proportion to an inductance, and at a high frequency, an alternating current is difficult to flow, so that an attenuation effect of high frequency signals can be obtained. Accordingly, when the above-described characteristic is to be obtained in a transformer, in the current situation, a structure using two cores each formed of a material having a high relative permeability (μ) of 10000 or higher is adopted. Note that in the following description, these two cores are referred to as a first core and a second core, respectively.
Here, the first core and the second core are arranged opposite to each other, and on a magnetic path formed between the first core and the second core, there is provided a gap (also referred to as an air gap) that is an empty space in which no magnetic material exists. Existence of such a gap enables attenuation of signals on the low frequency side.
Generally, this gap is provided, for example in an EP-type core, between a columnar leg portion of the first core and a columnar leg portion of the second core. It has been found that the smaller (narrower) this gap is, the better the obtained characteristic is, and thus in the current situation there is one having a dimension of approximately 22 μm. Incidentally, as the structure of a magnetic element having such a gap, there is one described in Patent document 1.
There also exists a structure such that a tape member made of resin or the like is attached on butting portions of the first core and the second core to thereby obtain a gap having a dimension of the thickness of the tape member between the first core and the second core. In this structure, the tape member exists at the boundary part between the first core and the second core, and the first core and the second core are joined with each other via this tape member.
Furthermore, as another technique to obtain a transformer having an inductance of desired value, the first core and the second core are formed of a material having a relative permeability (μ) of approximately 5000, and the first core and the second core are butted to each other without having a gap therebetween.
[Patent document 1] Japanese Patent Application Laid-open No. 2003-31422 (refer to Abstract, FIG. 1, and FIG. 5 to FIG. 8)
Meanwhile, among the above-described transformers, one combining the materials with a high relative permeability and having a narrow gap has a problem that it is difficult to form the narrow gap. Specifically, when forming a very narrow gap of 22 μm for example, it is difficult to accurately control dimensional precision. Also, when butting the first core to the second core, a butting error or the like occurs. Accordingly, it is difficult to provide the desired narrow gap between the first core and the second core. Further, when forming such a narrow gap with high precision, improvement in the aspect of precision is required, so that the processing cost thereof increases. The requirement to form the narrow gap also causes a problem that the time to produce a transformer becomes long, thereby decreasing production efficiency.
Further, in the structure having the tape member intervening between the first core and the second core, there is a problem that the tape member melts by heating. Specifically, when mounting the transformer on a board, a step accompanying heating such as soldering by reflowing is involved, but in such a heating step, the tape member in a thin film form having a dimension of the thickness of the above-described narrow gap melts easily. When the tape member thus melts, the gap dimension cannot be controlled accurately, and then the desired characteristic cannot be obtained.
Moreover, in the case where the first core and the second core are formed of the material having a relative permeability of approximately 5000, high frequency signals do not attenuate because there is no gap, so that the high frequencies are picked up as noise by a secondary side coil. In other words, when the first core and the second core are composed of the material having a relative permeability of approximately 5000, there is a problem that the characteristic as a band-pass filter is poor and a function thereof is not exhibited.
As described above, in the current situation, it is difficult to form a transformer having a desired characteristic equivalent to that of the transformer having a narrow gap as well as the advantage of high productivity that enables cost reduction.